The present invention is directed generally to an emergency capacitor in a contactless card.
The basic components of a contactless card system are a contactless reader and the contactless card. The contactless reader, also known as a PCD, includes an antenna electrically coupled to an electronic circuit. The contactless card, also known as a smart card, a tag, a PICC, or an RFID card, has an inductive antenna and an integrated circuit electrically coupled to the inductive antenna.
When the contactless card penetrates a transmission field of the reader, the reader antenna transmits to the contactless card a carrier signal, which generates a radio frequency (RF) field to supply the contactless card with power, and data, which is achieved by amplitude modulation of the carrier signal. In return, the contactless card transmits data by load modulating the carrier signal. This load modulated signal is detected by the reader antenna. The communication between the reader and the contactless card may be defined for example by ISO (International Organization for Standardization) 14443, Type A/B/C.
The ISO 14443 Type A communication protocol uses amplitude-shift keying (ASK) with a reader to contactless card modulation index of up to 100%. A single bit of data is coded as a field pause in the transmission. During the pause, the carrier field emitted by the reader antenna is reduced by the modulation index. At a modulation index of 100%, the carrier field is therefore turned off during a pause, which can last several microseconds. Since the emitted field of the reader antenna is also used to transfer energy to the contactless card, the contactless card is not supplied with energy during this time interval. An energy reservoir, such as an emergency capacitor, therefore has to be added to the contactless card to provide the energy consumed by the contactless card circuitry during the pause.
FIG. 2 shows a circuit diagram 200 of a portion of a contactless card including an emergency capacitor circuit. In cards implementing a load independent antenna interface, the emergency capacitor 246 is electrically coupled directly to a main rectifier 244 of the card. A serial regulator 252, which generates the internal supply voltage VDD, is electrically coupled to the output of the rectifier 244. The emergency capacitor 246 is charged by the main rectifier 244 with a charge current Icharge, and discharged by the main current source 248 with a discharge current Idischarge.
A voltage across the emergency capacitor 246 at node VDDRF equals the antenna voltage VLA/LB minus the voltage drop at the main rectifier 244. The voltage drop at the main rectifier 244 increases with increasing rectifier load current IAVG, so the voltage at the emergency capacitor 246 and the charge stored, is dependent on the chip-load current.
VDDMID shunt 250 discharges the emergency capacitor 246 when the voltage VDDRF at the emergency capacitor 246 is higher than a target regulation voltage VDDMID of the VDDMID shunt 250. The discharge current Idischarge of the emergency capacitor 246 is limited by the main current source 248. As the current consumed by main current source 248 is much larger than the current consumed by the VDD regulator 252 during a communication phase, most of the current from the emergency capacitor 246 is shunted through VDDMID shunt 250.
In normal operation, the contactless card antenna receives a carrier signal from the reader antenna generating a current on antenna 210. The main rectifier 244 is turned on, and the emergency capacitor 246, electrically coupled to the output of the main rectifier 244, is charged to the main rectifier output voltage at node VDDRF. During a 100% Type A field pause, the contactless card does not receive a carrier signal generating an induced voltage at the antenna 210. The main rectifier 244 turns off as the contactless card antenna voltage VLA/LB drops below the output voltage VDDRF of the main rectifier 244. During this time the current consumed by the contactless card circuitry is delivered by the emergency capacitor 246. Also, the emergency capacitor 246 will be discharged by the VDDMID shunt 250 in the VDD supply path during a 100% Type A pause. As a consequence the internal supply voltage VDD drops and a low voltage reset is triggered.